Guest mode isn't available with some IPv6 configurations on AirPort devices

To use Guest mode and IPv6 at the same time, you might need to add a value to the delegated prefix setting on your AirPort Extreme, AirPort Express, or AirPort Time Capsule.

Depending on how your IPv6 configuration is set, Guest network mode might not be available on your AirPort device:

When you specify a LAN prefix length for Manual Router or Tunnel mode that's equal to 64 bits, there are insufficient bits available to allow for a guest network. In these cases, IPv6 Guest network is unavailable.

If a prefix delegation isn't provided, your AirPort device assumes a prefix of 64. This also disables guest networking for IPv6.

You can use AirPort Utility to set an IPv6 prefix of 48 on your AirPort device. This allows you to use both IPv6 and Guest network mode at the same time.